London Calling: How Black and Asian Writers Imagined a City by Sukhdev Sandhu 512pp, HarperCollins, £20

In his introduction Sukhdev Sandhu expresses astonishment that a book like this hasn't been written before, but to anyone familiar with literary and academic life in Britain it's no surprise at all. I get half a dozen invitations a year from foreign universities to lecture on my fiction, and in my experience the most knowledgeable and well-read academics in the field are likely to be attached to universities in Germany, France, Italy or the US. In comparison critics in Britain continue to categorise domestic minority writing as "postcolonial", marginal - apart, of course, from a handful of headliners such as Hanif Kureishi, Salman Rushdie, Zadie Smith and now Monica Ali.

In the circumstances, London Calling is a radical and welcome departure, and Sandhu spells out his awareness of the context: "For too long black literature has been considered in extra-literary terms. It is treated primarily as a species of journalism, one that furnishes eyewitness accounts of sectors of British society to which mainstream newspapers and broadcasters have little access. Given that interest in black and Asian people tends to be at its highest when they are attacked, rioting or the subject of official reports documenting prejudice in some tranche of daily life, it is hardly surprising if black writing comes to be viewed as a kind of emergency literature, one that is tough, angry, 'real'. It was ever thus."

So far, so good. The first third of the book is occupied with 18th- and 19th-century writers such as Ukasaw Gronniosaw, sold on the Gold Coast for two yards of check cloth, whose ghost-written memoirs were published in Bath in 1772; the former slave and anti-slavery campaigner Olaudah Equiano (1745-1797); and Ignatius Sancho (1729-1780), friend and correspondent of the novelist Laurence Sterne, and the only black person among thousands who lived in England in the 18th century known to have been entitled to vote.

Sandhu then turns to the writing of Indian travellers, taking pains to draw out the differences between the experiences of aristocrats from the Indian subcontinent and temporarily beached (Asian) lascars or black ex-slaves from the Caribbean. Visitors such as Jhinda Ram and TN Mukharji displayed an "eagerness to savour all the political possibilities that London has to offer", he says. "The Indians in this chapter had the time, the contacts and the money to do so, and this not only distinguishes them from most of the other writers in this book, but also helps to scupper any theory about the relationship between colonials and the metropolis which assumes pre-20th-century contact was all disastrous."

Reading this, it struck me that I'd never come across a theory that tied together the experience of poor black slaves and Indian aristocrats. On the contrary, most commentators are quite clear that such people were treated differently and moved in altogether different circles. What Sandhu appears to be doing is inventing "theories" that he can knock down in order to demonstrate his freedom from "political correctness". It's a harmless enough strategy, much favoured by right-wing columnists, but it is also a method that can all too easily slide into contempt.

This tendency hardens as the book progresses. In a useful revisiting of Sam Selvon's reputation, Sandhu takes George Lamming to task for his somewhat patronising description of Selvon as a "peasant writer". In fact, as Sandhu points out, Selvon's techniques and language are sophisticated and complex. By the end of the chapter, however, he is using Selvon to take yet another swipe at political correctness. Spurning criticism of his characters, Sandhu writes approvingly: "Selvon prefers to reveal them in all their foolishness, their venality, fear, aspiration, confusion, passion, rather than have them stand tall and man the barricades in the war for racial liberation."

Like most non-Caribbean critics, he's missing the point. Selvon, a Trinidadian Indian like VS Naipaul, emerged from a region where blacks and Indians existed on opposite sides of a poisonous and often violent ethnic divide. In the context, there is something uncomfortable about the extent to which Selvon's humour is based on racist needling. Take one of the characters in Lonely Londoners, Fives, who gets his name when someone looks at him and remarks that he is as black as midnight, then looks again and says: "No. You more like Five Past Twelve." The innocence of the "peasant writer" of Lamming's imagination might have made the jibe excusable. For a writer of Selvon's acuteness it has a different meaning, and it's therefore no accident that the hallmarks of black identity in Selvon's work are foolishness, venality and a childlike gullibility. This "affection", as Sandhu calls it, is not so different from Naipaul's open hostility.

But the issue that most disappoints in the second half of the book is Sandhu's failure to get to grips with multiracialism in Britain. Discussing the era of Caribbean migration, he writes that "the docking of the SS Empire Windrush at Tilbury Docks in 1948 did not herald the beginning of multiracial Britain ... Long-established communities existed in Liverpool, Manchester, Cardiff, South Shields and Glasgow." Well of course they did. What Windrush actually symbolised was a period when the establishment of a black British population, and ensuing conflicts such as the Notting Hill riots, became a central issue of political and social life in Britain. The mid-1950s mark an important shift in London's identity, as the imperialist institutions that previously knitted together race, citizenship and nationality were dismantled - a process duly recorded by Selvon, Lamming, Naipaul and Andrew Salkey.

Sandhu makes great play of Caryl Phillips's portrayal of London as a "doomscape". "Is it the case," he asks plaintively, "that dire social circumstances need always be written up gloomily?" It depends, naturally, on who is writing and why. Sandhu compares Phillips's "gloom" with the brio of Hanif Kureishi, but the London into which Kureishi emerged had already been decisively altered by the presence of migrants and the possibility for transformation they created in the face of overwhelming hostility and isolation. Contrasting The Final Passage and My Beautiful Laundrette merely underlines the futility of comparing the experience of a black, migrant, urban family in the 1950s with that of the relatively prosperous British-born child of an English and Asian union, moving to London from the suburbs in the 1980s.

Thus the writers who approach London from such complex positions are shoehorned into the migrant saga, or ignored: there is no detailed consideration of writers such as Andrea Levy, Ferdinand Dennis or myself. Typically, Sandhu bookmarks me as a novelist "of distinction", quotes liberally from my non-fiction work about migration, but ignores the fiction set in the world of London's local and central government, media and art galleries, which clearly doesn't fit his preconceptions. Similarly, there is no consideration of the writers who experience London as part of family life or childhood. Nothing about the peculiar world of the housing estates. No Courttia Newland, or Brixton boys such as Alex Wheatle, Anton Marks et al. No mention either of the black writers who create characters from inside the enclaves of English professional life, such as Mike Gayle's (un)black London teacher, or Nicola Williams's black, female south London barrister.

As if conscious of these omissions, Sandhu's final chapter focuses on "improper London", with four authors from different periods: Salman Rushdie, Linton Kwesi Johnson, Victor Headley (of Yardie fame) and Robert Wedderburn, the 19th-century radical. It's an uncomfortable formula, partly because "improper London" is a banal notion that could include practically any of the authors he discusses. Particularly odd is his inclusion of Headley, whose work is based on the pretence of authenticity - and is therefore, to use Sandhu's formulation, "emergency literature" - and whose books are only marginally about London. Instead they chart a sort of fantasy Yardieland that could be transplanted to New York or Washington simply by changing the place names. And Yardie is simply bad writing. The prose plods and stutters, the characters are poorly imagined ciphers, the plots are endlessly derivative versions of the American mafia genre, filtered through a line of B movies. In much the same way, however, as real-life Yardie smugglers manipulate liberal arguments over immigration rules, the marketing machine behind Headley's books manipulates arguments about representation, suggesting that all criticism is PC propaganda designed to foster a virtuous black image. Sandhu seems to have swallowed all this whole, and instead of attempting literary judgments, compares Yardie to Rushdie's "refusal to sentimentalise or emolliate illicit aspects of black London".

This seems a thoroughly weasel comparison, and suggests that - contrary to the brave claims in his introduction - there is no distinction to be made between the literary qualities of black writing. It avoids, also, any judgments about the dangerous consequences in the real world of the literary fetishisation of "black crime". It's a rather sad conclusion to a thesis that promised so much.

Mike Phillips's London Crossings: A Biography of Black Britain is published by Continuum.